Materials testing apparatus is useful to measure the stress strain and yield strength of materials under carefully controlled conditions. When compression or tensile strength testing is done it is generally desireable to minimize the bending strains in the sample.
In tensile testing, the sample is generally placed between two pull rods while one rod remains fixed and the other experiences a load. Bending strains arise at the start of the test when the pull rods are not in perfect coaxial alignment or the sample is not in coaxial alignment with the pull rods.
The load is generally applied by use of a lever. The lever separates the pull rods thus stretching the sample, but, at the same time the sample stretches, the lever action moves the pull rods out of their original alignment. In this way dynamic bending strains arise in the sample.
Prior inventions have reduced the initial bending strain by interposing an alignment fixture between the load and the moving pull rod in the load train. The bending strain is measured on a dummy sample with an alignment extensiometer and the alignment fixture is used to align the pull rods in a fixed position so that the strains measured by an alignment extensiometer are equal on all sides of the dummy sample. The extensiometer is then removed and the dummy sample replaced with the test sample.
Prior inventions have also reduced some of the initial bending strain and some of the dynamic strain by interposing a free moving swivel, (lower alignment coupling), between the stationary pull rod and the fixed base and a second free moving swivel (upper alignment coupling) between the moving pull rod and the load train. As the load is applied or increased the couplings freely move so as to relax the bending of the sample until an equilibrium is established between the bending of the sample and the swiveling of the coupling.
Alignment fixtures and free moving alignment couplings do not eliminate bending strains. The present invention serves to reduce the bending strain and, if desired, to subject to the sample to a known bending strain.